In recent years, various kinds of bio-sensor based on the specific catalytic action of enzyme have been developed, and particularly, their application to clinical inspection field is being tried. At the present day wherein inspection items and the number of specimens are increasing, bio-sensors which enable rapid and high-precision measurement are demanded.
A case of glucose sensor will be taken as example. In these days of remarkable increase of diabetes, for measurement and control of blood sugar in blood, there is a demand for sensors applicable to tests on whole blood, because it takes a very long time to centrifuge blood and measure the separated plasma as conventionally carried out. There is provided a simple sensor like the test paper used for urinalysis, and this sensor comprises a stick-form support and a carrier put on the support containing an enzyme which will react with glucose only and a dye which will change itself on enzyme reaction or by the product of the reaction. Blood is added to the carrier, and a change in the dye after a definite period of time is measured visually or optically. But, the measurement is largely disturbed by dyes in blood, so that its precision is low.
The multi-layer analytical carrier was thus proposed as shown in FIG. 1 (Japanese Utility Model Opening (KOKAI) No. 178495/1979). This carrier has a laminated structure wherein a reagent layer 2, developing layer 3, water-proof layer 4 and filter layer 5 are placed one upon another in this order on a transparent support 1. When a blood sample is dropped down to the carrier, the blood is first freed from its solid components such as red blood cells, blood platelets, etc. on the filter layer 5, uniformly diffuses into the developing layer 3 through small pores in the water-proof layer 4 and comes into reaction in the reagent layer 2. After completion of the reaction, light is passed through the transparent support in the direction of an arrow to measure the substrate concentration spectroanalytically. Although this carrier is complicated in structure as compared with the conventional sample stick-form carrier, an improvement in precision was attained by removing blood cells, etc. But, it takes a long time for the penetration and reaction of blood, so that there was a necessity to put the water-proof layer 4 for preventing the sample from drying and to incubate at high temperatures for accelerating the reaction, which caused a problem of the apparatus and carrier becoming complicated.
Thus, an electrode method comprising combining enzyme reaction with electrode reaction was developed as a simple method of good precision. Since there is no disturbance of colored substances in the electrode method, the liquid sample can be used as it is without a necessity for pre-treatment, and therefore, the measurement became simple and precision was also improved. A case of glucose sensor will be taken as example. A flow-type sensor was developed as shown in FIG. 2 in which a liquid sample, e.g. blood or urine, is added to the sensor while applying a definite voltage to a glucose oxidase immobilized electrode 6 and passing a buffer solution 7 through a conduit 8 made of insulating materials such as acrylic resin; glucose in the sample reacts with the immobilized glucose oxidase to produce hydrogen peroxide which is then oxidized at the electrode 6 to generate a current; and the glucose concentration of the sample can be detected by measuring the strength of the current. The sensor of this type enables as many specimens as 200 to 300 per hour to be measured with rapidity and high precision, but it had a problem that the apparatus becomes large in size. The so-called batch-type sensor as shown in FIG. 3 was thus developed in which a glucose oxidase immobilized electrode 11 is placed in a vessel 10 which is then filled with a buffer solution 12, and a liquid sample is added to the solution while stirring with a stirrer. The size of apparatus could be made fairly small by using this type, but there occurred problems that a stirrer is essential, and that foaming and turbulent flow are caused by stirring to exert an adverse effect on the precision. Also, there was a necessity to exchange the buffer solution and sometimes wash the electrode, and besides, because of the liquid sample being diluted with the buffer solution, precision was required for the amounts of buffer solution and liquid sample.
Thus, for simple measurement, a dry measurement-form sensor requiring no stirring apparatus is demanded, and besides a high precision is required.